For measuring the frequency of light to high accuracy, the light being measured is caused to interfere with other light, and an electrical signal of the optical beat frequency generated is measured, by way of performing heterodyning detection. The frequency band of light that may be measured by this heterodyning detection is limited by the frequency band of a light receiving device as used in the detection system, and is on the order of tens of GHz.
On the other hand, for keeping pace with the recent development in opto-electronics, the frequency band of light that may be measured needs to be increased further in order to carry out light control for frequency multiplexed communication or in order to measure the frequency of absorption rays distributed over a wide range.
For meeting the demand for increasing the frequency band that may be measured, a broadband heterodyning detection system, exploiting an optical frequency comb generator, such as is disclosed in Japanese Laid-Open Patent Publication 2003-202609, has been proposed. This optical frequency comb generator generates a number of comb-like sidebands, spaced apart at equal intervals on the frequency axis. The frequency stability of the sidebands is approximately equal to that of the incident light. These sidebands are subjected to heterodyning detection with the light under measurement to construct a broadband heterodyning detection system extending over a range of several THz.
FIG. 1 shows the principle of the configuration of a conventional optical frequency comb generator 3.
This optical frequency comb generator 3 uses an optical resonator 100 including an optical phase modulator 31 and reflective mirrors 32, 33 arranged facing each other with the optical phase modulator 31 in-between.
With this optical resonator 100, light Lin incident via reflective mirror 32 with a small transmittance is caused to be resonant between the reflective mirrors 32, 33 and part of light Lout is radiated via reflective mirror 33. An optical phase modulator 31 is formed by an electro-optical crystal for optical phase modulation, the refractive index of which is changed by application of an electrical field. The light passing through the optical resonator 100 is subjected to phase modulation by an electrical signal of a frequency fm applied to the electrode 36.
In this optical frequency comb generator 3, an electrical signal, synchronized with the time of reciprocation of light through the optical resonator 100, is input from the electrode 36 to the optical phase modulator 31. By so doing, it is possible to apply phase modulation deeper by tens of times than in case the signals traverse the optical phase modulator 31 only once. It is possible to generate hundreds of higher order sidebands, with the frequency intervals fm between neighboring sidebands all being equal to the frequency fm of the input electrical signal.
The conventional optical frequency comb generator is not limited to the above-described bulk type configuration. For example, a waveguide path type optical frequency comb generator 20, employing a waveguide path, may also be used.
This waveguide path type optical frequency comb generator 20 is formed by a waveguide path type optical modulator 200. The waveguide path type optical modulator 200 is made up of a substrate 201, a waveguide path 202, an electrode 203, a light incident side reflective film 204, a light exiting side reflective film 205 and an oscillator 206.
The substrate 201 is prepared by slicing a large-sized crystal of LiNbO3 or GaAs, 3 to 4 inch in diameter, grown by, for example Czochralski method, in the form of a wafer.
A waveguide path 202 is provided for propagating light thereon. The refractive index of the layer, which forms the waveguide path 202, is set so as to be higher than that of the other layers, such as substrate layer. The light incident on the waveguide path 202 is propagated as it undergoes total reflection on the boundary surface of the waveguide path 202.
The electrode 203, formed of a metal material, such as Al, Cu, Pt or Au, applies an electrical signal of a frequency fm, supplied from outside, to the waveguide path 202. The direction of propagation of light through the waveguide path is equal to the proceeding direction of the modulating electrical field.
The light incident side reflecting film 204 and the light exiting reflecting film 205 are provided for causing resonance of light incident on the waveguide path 202. The incident light is set in the resonant state by light bouncing back and forth through the waveguide path 202. An oscillator 206 is connected to the electrode 203 to supply the electrical signal with a frequency fm.
The light incident side reflecting film 204 is arranged on the light incident side of the waveguide path type optical modulator 200. On this light incident side reflecting film falls the light of a frequency ν1 from a light source, not shown. This light incident side reflecting film 204 reflects light reflected by the light exiting side reflecting film 205 and which has traversed the waveguide path 202.
The light exiting side reflecting film 205 is arranged on the light exiting side of the waveguide path type optical modulator 200, and reflects the light which has traversed the waveguide path 202. The light exiting side reflecting film 205 also radiates to outside a preset portion of the light which has passed through the waveguide path 202.
In the above-described waveguide path type optical frequency comb generator 20, an electrical signal, synchronized with the time of light reciprocation in the waveguide path 202, is supplied from the electrode 203 to the waveguide path type optical modulator 200 This renders it possible to apply phase modulation deeper by tens of times than if the electrical signal is passed through the optical phase generator only once. By so doing, an optical frequency comb may be generated which has a number of sidebands extending over a wide range, as in the bulk type optical frequency comb generator. The frequency interval between neighboring sidebands unexceptionally becomes equal to the frequency fm of the input electrical signal.